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Radar Radar equipment used in ships, aircraft, and airfield control towers employs pulses of very short radio waves, for the purpose of finding the distances from the transmitter of various objects, e.g., coastline and building outlines together with other ships or aircraft. A rotating (scanning) aerial sends out a series of high-frequency radio pulses which are received back as […]

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Origin and sources of electromagnetic waves The whole range of electromagnetic radiation pours on to the earth from the sun and other heavenly bodies in outer space. Those frequencies which are stopped by the earth’s atmosphere have been detected by instruments in man-made satellites. Otherwise some of the main sources on earth are given in the table on […]

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The inverse square law for electromagnetic waves Let us suppose that an area of 1 m- which forms part of the surface of a sphere of radius I m receives wave energy from a point source placed at the centre S (Fig. 26.20). If P is the energy passing through this unit area in joules per second, […]

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Attenuation The inverse square law for radiation as presented above holds strictly for waves through free space from a point source. If the radiation passes through a material medium of some kind then the law is modified by the fact that some of the wave energy is progressively absorbed. The loss of power from this cause is described […]

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Electromagnetic waves When the wave theory of light first gained general acceptance it was considered that light waves were conveyed through a transparent elastic medium which filled the whole of space, even a vacuum. This substance was called the ether. A further step forward was made in 1845, when Michael Faraday showed that, under certain conditions, light waves passing […]

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The Sl unit of frequency In recognition of the importance of Hertz’s researches into electromagnetic waves, his name has been given to the unit of frequency. The SI unit of frequency is called the hertz (Hz) and is equal to a frequency of 1 cycle per second. The term hertz is not restricted to wave frequencies only but […]

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Higher order spectra from a diffraction grating We have seen that reinforcement occurs for light diffracted from adjacent slits in a diffraction grating at an angle 8 given by, Reinforcement will also occur when the angle of diffraction is such that the path difference for light from adjacent slits is 2}” 32, and so on. We therefore may […]

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To measure the wavelength of light by a simple diffraction grating experiment Fig. 26.18 shows how the ordinary type of ray-box used in elementary laboratories may be used in conjunction with a diffraction grating and a second cylindrical lens to form a spectrum of white light and to measure the wavelength of any particular colour in it. The cellulose […]

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Formation of spectrum by diffraction grating If the monochromatic light source used previously is replaced by white light, then each wavelength in the white light will be diffracted at its own particular angle 8 which satisfies the equation A= d sin e The net result is the formation of a continuous line of coloured images of the slit in the […]

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Relation between angle of diffraction and wavelength Fig. 26.17 shows the usual arrangement for a diffraction grating experiment. A parallel beam of light is incident on the grating from an illuminated slit S placed at the principal focus of a converging lens L!. The first order diffracted beam from the grating enters a second lens L2 which […]